This invention relates to the plasma processing of an object, and, more particularly, to nondestructively assessing the plasma processing operating conditions required to achieve preselected treatment characteristics of such an object before plasma processing the object itself.
Plasma processing encompasses a family of techniques that may be used to alter the surface properties of objects in a beneficial manner. To cite an example, certain types of tooling, such as some stamping and pressing dies, are desirably made of a soft material that is readily machined or formed. This allows the tooling to be manufactured relatively inexpensively. Nonmetallic materials made of epoxy and modified epoxy are candidates for use as such too ling. On the other hand, the working surface of the tooling should be as wear resistant as possible so that the dimensions do not change as successive parts are manufactured.
The sur face softness of the nonmetallic tooling that permits easy machining results in a short useful life of the tool. For production applications, an epoxy tool cannot be used without a treatment to make the surface more wear resistant. It has been found that several types of ion and electron treatments of the surfaces of polymers such as epoxies and the surfaces of metals can significantly increase their useful lives. Such treatments include, for example, ion implantation, ion deposition and ion mixing, and electron bombardment.
Ion implantation is a process wherein ions are accelerated by an electrostatic potential to impact a surface. The energy of the ions causes them to be imbedded beneath the surface. A sufficiently high dose or concentration of implanted ions can significantly increase the hardness of the surface layer. In ion mixing, a thin (about 300-600 Angstrom) material layer is first deposited onto a surface to be treated, and then the composite surface is ion implanted to mix the deposited material atoms into the surface. Ion deposition is similar to ion mixing except that the accelerating energies of the ions are lower, with the result that the ions are deposited simultaneously with the deposited material layer upon the surface being treated rather than imbedded beneath its surface. In electron bombardment of polymeric surfaces, energetic electrons (100-20,000 volts or more) are accelerated by an electrostatic potential to cause them to impact into the surface to be treated. The use of these treatments produce improved hardness and wear characteristics of the substrate.
Such treatments are traditionally accomplished by accelerating a beam of electrical charge carriers (ions or electrons) using electrostatic acceleration electrodes. This approach, while effective, has the drawback that it is difficult to uniformly treat a large, three-dimensional, irregularly shaped object such as a typical automotive tool that may measure 3 feet by 5 feet by 1 foot in size. The beam of charge carriers must be moved slowly over the entire surface, including those areas that may be rather inaccessible such as deep holes or recesses, or for protrusions that stick out from the surface. In these cases, the object must be manipulated to uniformly treat the area with a beam. For large tools or for large numbers of smaller objects having such surface features, surface treatment using ion or electron beams becomes prohibitively slow and expensive.
An alternative approach that has promise for ion implanting, ion depositing, or ion mixing of the surfaces of large objects is plasma ion implantation (PII), described in U.S. Pat. No. 4,764,394. A plasma of ions is created adjacent to the surface of the object to be implanted, and the object is electrostatically charged to a potential opposite to that of the ions. For example, if positively charged nitrogen ions are to be implanted, the object is negatively charged using repetitive voltage pulses of typically about 100,000 volts or more. The nitrogen ions are attracted to the surface of the object by this accelerating potential and driven into the surface and sub-surface regions of the substrate. Plasma ion implantation has the advantage that the plasma of ions provides a source that is distributed around the entire surface area of the object, and uniform implantation over the entire surface area is simultaneously achieved.
Plasma ion mixing may be performed in a similar manner, except that first a thin layer of material is deposited onto the surface prior to implantation. Plasma ion deposition is also performed in a similar manner, except that the accelerating potential is typically lower so that the ions and material layer deposit upon the surface instead of being driven into the sub-surface layers. It is possible to achieve electron bombardment of a surface using plasma techniques similar to those used in ion implantation. Plasma ion implantation, plasma ion deposition and plasma ion mixing, and electron bombardment, and related techniques, are collectively termed "plasma processing".
When plasma processing of an irregularly shaped object is to be performed, care must be taken to achieve the desired treatment of the object at all locations on the surface of the object. The irregular shape of the object makes it difficult, if not impossible, to design the plasma processing apparatus and system from first principles, so that exactly the desired treatment at each location is achieved.
At the present time, a trial-and-error approach is used to determine the treatment characteristics of such objects. A "dummy" object is prepared with the shape and size of the object to be plasma processed. A number of metallic test coupons are affixed to various locations on the surface of the dummy object. The dummy object is plasma processed by the intended approach, and the coupons are analyzed to determine the treatments at each coupon location. The total treatment and spatial distribution are then mapped. If the total treatment and spatial distributions are not as desired, the plasma processing system is altered as necessary, and the checking process repeated. Once the final processing parameters have been identified, the dummy object is replaced by the object to be plasma processed and the processing is performed.
This technique for assessing treatment characteristics has major drawbacks. It is slow and tedious. The coupons must be individually analyzed using sophisticated analysis techniques such as SIMS (Secondary Ion Mass Spectroscopy), which may not be readily available in some instances. For a typical case of a commercial-scale automotive die to be plasma ion implanted, 20-50 coupons are required and the analysis can easily require a day to complete. A duplicate dummy object must be manufactured to at least approximately the dimensions of the object to be implanted, which can be a considerable expense because the object to be implanted may otherwise be a one-of-a-kind piece. As a variation of this approach, the ion dosage characteristics can be assessed on the object itself, but this is impractical for large-scale objects that cannot fit inside a SIMS apparatus. Furthermore, if the optimum dosage is accidentally exceeded, it could ruin the object for its intended purpose.
There is a need for an improved approach to determining the operating parameters required to achieve the desired treatment characteristics of large-scale, irregularly shaped objects, or large numbers of smaller objects, in plasma processing operations. The present invention fulfills this need, and further provides related advantages.